The present invention relates to a semiconductor laser device composed of a group III–V compound semiconductor and, more particularly, to a semiconductor laser device capable of a high-output and low-voltage operation and to a method for fabricating the same.
Because of their ability to record information at an extremely high density, digital versatile disk (DVD) devices have achieved rapid widespread use in the fields of personal computers and audio-video equipment. In particular, growing expectations have been placed on further proliferation of writable or rewritable DVD devices as, e.g., external large-capacity memory devices (such as so-called DVD-R and DVD-RAM) or as next-generation video recorders (so-called DVD recorders) which are replacements for video tape recorders.
As a pick-up light source used in such a writable or rewritable DVD device to enable reading or rewriting of data, a semiconductor laser device which emits a red light beam at a wavelength of 650 nm has been used. To increase a writing speed in a DVD device, an operation with a high output over 100 mW has been required of a semiconductor laser device in recent years.
In the semiconductor laser device which emits the red light beam, an AlGaInP-based compound semiconductor which is a group III–V compound semiconductor containing at least one of aluminum (Al), gallium (Ga), and indium (In) as a group III element and containing phosphorus as a group V element is used in an active layer and a cladding layer.
FIG. 8 shows a cross-sectional structure of a conventional semiconductor laser device composed of an AlGaInP-based compound semiconductor. As shown in FIG. 8, the conventional semiconductor laser device is constituted by: an n-type substrate 101 made of gallium arsenide (GaAs); an n-type cladding layer 102 made of n-type AlGaInP; an active layer 103 composed of a multiple quantum well layer 103a consisting of well layers made of GaInP and barrier layers made of AlGaInP, which are alternately stacked, and upper and lower optical guide layers 103b made of AlGaInP and formed with the multiple quantum well layer 103a interposed therebetween; a first p-type cladding layer 104 made of p-type AlGaInP; an etching stopper layer 105 made of p-type GaInP; a ridge-shaped second p-type cladding layer 106 made of p-type AlGaInP; a first contact layer 107 made of p-type GaInP; a first current blocking layer 108 made of n-type AlInP and formed to sandwich the second p-type cladding layer 106; a second current blocking layer 109 made of n-type GaAs; and a second contact layer 110 made of p-type GaAs.
An n-side electrode 111 made of a metal material is formed on the lower side of the n-type substrate 101 to make ohmic contact therewith, while a p-side electrode 112 made of a metal material is formed on the upper side of the second contact layer 109 to make ohmic contact therewith.
In the conventional semiconductor laser device, current components injected from the p-side electrode 112 by the application of a specified voltage to the n-side electrode 111 and to the p-side electrode 112 are confined by respective pn junctions between the ridge-shaped second cladding layer 106 and the first current blocking layer 108 and between the first and second current blocking layers 108 and 109 to reach the active layer 103 through the second p-type cladding layer 105 and the first p-type cladding layer 104 and cause radiative recombination in the active layer 103, so that a laser beam with a wavelength of about 650 nm corresponding to the band gap of the well layer is emitted. In this case, a multilayer structure composed of the second p-type cladding layer 106, the first p-type cladding layer 104, the active layer 103, and the n-type cladding layer 102 serves as a resonator.
To enable the conventional semiconductor laser device to perform a high-output operation, it is important to heavily dope the first p-type cladding layer 104 with a p-type impurity. If the impurity concentration in the first p-type cladding layer 104 is low, electrons injected from the n-side electrode 111 into the active layer 103 overflows from the active layer 103 to the first p-type cladding layer 104. The overflow of electrons reduces a threshold current and an operating current so that a sufficient output is not obtained.
If the first p-type cladding layer 104 is heavily doped with a p-type impurity, however, the p-type impurity is diffused into the active layer 103 to form a nonradiative recombination center so that the temperature characteristic of the semiconductor device deteriorates and the reliability thereof is degraded.
The provision of an undoped spacer layer between the active layer 103 and the first p-type cladding layer 104 prevents the diffusion of the p-type impurity into the active layer 103 even if the p-type impurity concentration in the first p-type cladding layer 104 is increased.
By doping the first and second p-type cladding layers 104 and 106 with magnesium (Mg) with a low diffusion coefficient, the p-type impurity becomes less likely to be diffused into the active layer 103 so that the p-type impurity concentration in each of the semiconductor layers is increased.
In the conventional semiconductor laser device, however, currents are confined in the ridge-shaped second p-type cladding layer 106, pass through the first p-type cladding layer 104, while being diffused therein, and reach the active layer 103. Consequently, the currents are diffused not only in the portion of the first p-type cladding layer 104 underlying the second p-type cladding layer 105 but also in the other region of the first p-type cladding layer 104. In the region of the active layer 103 other than the portion thereof underlying the second p-type cladding layer 105, therefore, a sufficient current density for the oscillation of a laser beam cannot be obtained.
Thus, the conventional semiconductor laser device described above has a problem that a high output cannot be obtained therefrom because ineffective currents resulting from the diffusion of currents in the cladding layer formed on the active layer reduce a luminous efficiency and increase a threshold current as well as an operating current.
If the impurity concentration in the first p-type cladding layer 104 is increased to suppress the overflow of electrons, the electric conductivity of the first p-type cladding layer 104 is increased so that a current is more likely to be diffused even in a direction parallel with the first p-type cladding layer 104 and the ineffective currents are increased disadvantageously.